Autism spectrum disorders (ASD, including Autism, Pervasive Developmental Disorder-Not Otherwise Specified, Asperger syndrome) are characterized by significant impairment in communication and emotional understanding and expression, as well as a limited behavioral repertoire with restricted interest and activities. Although early diagnosis and intervention are critical, ASD is rarely diagnosed before three years of age. This is problematic given evidence: 1) of prenatal brain abnormality in autism (Rodier et al, 1996; Bauman & Kemper, 1994; Bailey et al., 1998); 2) that symptoms of autism manifest before 24 months of age; and 3) of the importance of early intervention to capitalize on the neuroplasticity of the young brain. Particular attention should be paid to a very high-risk group: the young siblings of autistic children (hereafter, autism sibs). Family history studies indicate that autism sibs are at increased risk for ASD and communicator disorder, implying a need for screening during infancy. Young autism sibs provide researchers with an important opportunity to learn about the early manifestation and developmental trajectory of ASD, the relationship between social and language domains in the development of typical and atypical populations, the genetics of autism, and neuropsychological bases brain development in ASD and the broader autism phenotype. The proposed prospective study will enroll 340 autism sibs, tested at 14, 24, and 36 months of age, to address these specific aims: 1) To identify behavioral profiles that enhance diagnosis of ASD in the first two years of life of infant siblings of children with autism. 2) to test hypotheses about the relationship between joint attention, shared positive affect, and communication in infant siblings later diagnosed with ADS (at 3 years of age) and in those siblings without ASD. In addition, a sample of infants at no known risk for autism (having no developmental disorder) will be included as a basis of comparison of joint attention, communication, and affect development in non-ADS autism sibs.